1. Field of the Invention
This invention relates to an apparatus which detects, by use of a non-contact type probe rotating around an object to be inspected, flaws on the outer periphery of a product, such as a steel bar, wire or steel pipe, round in section and manufactured by the hot rolling process.
2. Description of the Prior Art
Generally, a bar in coil or a wire of 50 mm or less in diameter is coiled immediately after rolled so that when flaw detection is performed in a cold condition, the coiled object need once be uncoiled for flaw detection and thereafter be recoiled. Therefore, it is efficient and desirable to detect flaws prior to coiling the object, in other words, in a hot condition just after the rolling process.
For inspecting the bar in coil or the like during the hot rolling process, an encircling coil type eddy current inspection method (electromagnetic induction flaw detection method) has hitherto been put into practice, which is a self-comparison method (obtaining self-impedance difference between two coils provided lengthwise of object to be inspected), whereby a short independent flaw, such as a scab or a roll mark, is detectable but harmful flaws existing lengthwise of the object are not so.
While, in order to detect flaws extending lengthwise of a long object, such as bar in coil, an eddy current inspection apparatus of rotary probe type has hitherto been put into practice, which rotates a probe coil around the bar in coil or the like at high speed in the cold condition, the apparatus picks up variation in impedance of the probe to detect the surface flaws, the variation in impedance being caused not only by the surface flaws on the object to be inspected but also by its quality, size and lift-off (a distance between the surface of object to be inspected and the probe coil), whereby when the size and quality of object to be inspected are different, the probe coil need be adjusted of its sensibility and it is important to scan the object keeping the lift-off constant.
The conventional eddy current inspection apparatus of rotary probe type used during the cold rolling keeps the lift-off constant by either one or combination of the following methods:
(1) to fix by pinch rolls the object to be inspected to thereby coincide the axis of object to be inspected with the axis of rotation of probe, and PA1 (2) to keep the probe in contact with the surface of the object to be inspected and follow the object. PA1 (1) a signal caused by lift-off variation is superposed on the flaw signal so that both the signals are not distinguishable from each other, and PA1 (2) the lift-off variation changes the flaw signal itself.
FIG. 1 is a view exemplary of the eddy current inspection apparatus of rotary probe type, which adopts the above method (1). In detail, a rotary drum 202 driven by a motor 201 is disposed within a transfer zone of an object 10 to be inspected, two probe coils 203 are mounted at the inner surface of the rotary drum 202 and opposite to each other, at both axial sides of rotary drum 202 are disposed sleeves 204, 204 each of very hard metal and larger by about 0.1 to 0.2 mm in diameter than the object 10 to be inspected, the sleeves 204, 204 being concentric with the rotary drum 202, and pinch rolls 205, 205 holding the object 10 are mounted to the entrance and exit sides of the flaw detection apparatus in relation of being contactable with or movable away from the object 10 by air cylinders 206, 206 respectively, so that a motor 201 drives the rotary drum 202. Hence, the object 10 is rigidly fixed by the pinch rolls 205, 205 and guided by the sleeves 204, 204, whereby the object 10 to be inspected can be suppressed as much as possible of vibrations and eccentricity with respect to the rotary drum 202 and probe coils 203, 203.
Such flaw detection apparatus of rotary probe type, however, has many difficulties even when intended to be applied to the object to be inspected in the hot rolling, especially when applied in-process to the finish rolling or just after the finish rolling of bar in coil. In other words, since the rigidity of the object to be inspected under the hot rolling is small, it is impossible to rigidly hold the object by the pinch rolls because there is a fear of producing flaws on the surface of object and of deforming the object, and also the object cannot be suppressed completely of its vibrations, whereby the probes strike the object to be inspected to result in a fear of causing the flaws. Since guidance of the sleeves 204, 204 is impossible for the similar reason, it is difficult to coincide the axis of rotation of the drum 202 with the axis of object to be inspected, especially to obtain the axis thereof because the object is not round properly in section. The product of bar in coil or wire, even when uniform as well as different in the size, gradually varies in its pass line due to a different number of rolling times so that the axis of object to be inspected need be coincident every time with the axis of rotation of the probe.
Also, the aforesaid method (2) to keep the probe in contact with the surface of the object to be inspected and follow the object has a grave fear of causing flaws on the object in the hot rolling.
Furthermore, there are many factors, such as vibrations of a roll stand for the rolling or of a winding up mechanism, which induce vibrations on the object under the hot rolling, and the object itself is of less rigidity so as to have a grave fear of deflection between the pinch rolls, resulting in that the eddy current inspection apparatus of rotary probe type has been difficult to put into practice under the hot rolling.
Thus, the difficulty of keeping the lift-off constant will suggest the correction of detection signal by measurement of lift-off. Regarding the relation between the detection signal and the lift-off variation, there are two problems as follows:
Hence, the detection capability is remarkably deteriorated, whereby it is required to suppress a lift-off variation signal (a signal component caused by the lift-off variation) and also correct the flaw signal corresponding to the lift-off variation.
The phase discrimination method or the frequency discrimination method is well known as the aforesaid suppression of lift-off variation signal, the phase discrimination method phase-detecting the detection signal by the probe coils to thereby suppress noises and discriminate the flaw signal. FIGS. 2-(a) and -(b) are vector diagrams of signals from coils. In a case where the signal A caused by lift-off variation is different in phase from the flaw signal B as shown in FIG. 2-(a), the lift-off variation signal A intended to be suppressed is selected of the component of phase perpendicular to the direction of signal A (in the direction X) to thereby pick up the flaw signals B to be inspected. However, the flaw signal B and lift-off variation signal A do not always appear in different phase as shown in FIG. 2-(a), but often in a slight difference as shown in FIG. 2-(b), in which there is no effect in the suppression of lift-off variation signal A.
On the other hand, the frequency discrimination method is to suppress a not-desired signal (lift-off variation signal) by means of a difference between the same and the signal to be inspected (the flaw signal), but there is no effect for similar frequency components of both the signals.
For suppression of the signal difficult to suppress by such phase discrimination or frequency discrimination method, a multifrequency method is well known, which is to apply to an inspection coil currents of different frequencies in mixture, separately detect the signals each of frequency component, and compute a plurality of signal outputs obtained, thereby separating undesired signals.
FIG. 3 is a block diagram of the well-known multifrequency eddy current inspection apparatus used for the multifrequency method, in which reference numeral 211 designates an oscillator of frequency f.sub.1 (e.g., 100 kHz), 212 designates an oscillator of that f.sub.2 (e.g., 500 kHz), the outputs of both oscillators 211 and 212 being mixed in a mixer 213 and applied to detection coils 225 and 226 disposed in the self-comparison system through an impedance bridge 214 so that signals representing impedance variations in the coils 225 and 226 are given to tuned amplifiers 219 and 220 from the impedance bridge 214. The signals are amplified in synchronism with the frequencies f.sub.1 and f.sub.2 by the tuned amplifiers 219 and 220 respectively, the outputs therefrom, when viewed in the vector diagram, have the contents as shown in FIG. 2-(b). The outputs of tuned amplifiers 219 and 220 are given to phase sensitive detectors 221, 222 and 223, 224 respectively, the phase sensitive detector 221 or 223 being given, as the phase reference signal, the signal produced from an output of oscillator 211 or 212 given to a phase shifter 215 or 216, the output therefrom being given to a .pi./2 phase shifter 217 or 218 for shifting the phase by .pi./2 so that an output from shifter 217 or 218 is given to a phase sensitive detector 222 or 224 as the phase reference signal. The phase shifter 215 or 216, for example as shown in FIGS. 2-(a) and -(b), adjusts the signal A perpendicularly to the axis X, in other words, identically with the axis Y. Accordingly, a X-component (a resistance component) X.sub.1 or X.sub.2 of detection signal obtained by the frequency f.sub.1 or f.sub.2 is obtained by the phase sensitive detector 221 or 223 and a Y-component (a reactance component) Y.sub.1 or Y.sub.2 of the detection signal obtained by the frequency f.sub.1 or f.sub.2 is obtained by the phase sensitive detector 222 or 224.
Reference numeral 230 designates an analog signal-computing unit comprising phase rotators 231 and 232 for phase-rotating signals X.sub.2 and Y.sub.2 at an equal angle, amplifiers 223 and 234 for amplifying the outputs of phase rotators 231 and 232 in equal gains respectively, and differential amplifiers 235 and 236 which are given the outputs X.sub.2 ' or Y.sub.2 ' and X.sub.1 or Y.sub.1 of amplifiers 233 and 234 respectively, thereby obtaining the outputs x and y of differential amplifiers 235 and 236 respectively.
FIG. 4-(a) is a vector diagram of X.sub.1 and Y.sub.1, and FIG. 4-(b) is the same of X.sub.2 and Y.sub.2, in which the lift-off component A.sub.2 shifts from the axis Y.sub.2. The phase rotators 231 and 232 at the signal computing unit 230 are operated to rotate the phase and the amplifiers 233 and 234 are operated to equalize amplitude of signal A.sub.2 to that of signal A.sub.1, thereby obtaining signals A.sub.2 ' and B.sub.2 ' as shown in FIG. 4-(c) and giving them to the differential amplifiers 235 and 236 respectively. Since the differential amplifiers 235 and 236 output a difference between two input signals, a difference A.sub.1 -A.sub.2 ' between the lift-off components becomes a signal "a" of minute level as shown in FIG. 4-(d) and when the flaw signal exists, a difference "b" between B.sub.1 and B.sub.2 (expressed in vector) is obtained.
On the hand, a change in the flaw signal following the lift-off variation in the aforesaid item (2), as shown in FIG. 5, abruptly attenuates following an increase of lift-off, whereby in condition of lift-off variation, some means should be taken to detect the lift-off and correct the signal output.
The multifrequency method can almost suppress the signal caused by the lift-off variation, but it is extremely difficult to accurately detect the lift-off variation and correct the flaw signal. The first reason is as follows. It is difficult to detect the lift-off variation with accuracy. For example, it is impossible to measure the lift-off variation by a distance detector which is provided independently from the probe so as to detect the distance between the probe coil and object to be inspected. A contact system with a differential transformer or the like can not follow at its contact end after the object moving at high speed. A measuring device of eddy current type, when in use, is affected by flaws or the like, the detection result becoming a mixture of factors of both the lift-off and flaws. Second reason is as follows. The lift-off variation is divided roughly into two forms which should be corrected separately from each other. FIGS. 6-(a) and -(b) are schematic views explanatory of configurations of lift-off variation in the self-comparison system when the object 240 to be inspected is round in section like bars in coil, FIGS. 7-(a) and -(b) are schematic views explanatory of the same of self-comparison system when the object 240 is a plate-like object, such as a steel plate. FIGS. 6-(a) and 7-(a) show the object 240 moving as a whole toward the probe coils 225 and 226, FIGS. 6-(b) and 7-(b) show the object 240 moving other directions, and FIGS. 8-(a) and -(b) show detection patterns of flaw signal in FIGS. 6-(a) and 7-(a) and FIGS. 6-(b) and 7-(b) respectively. The flaw signal, which usually appears in origin symmetry as shown by the solid line in FIGS. 8-(a) and -(b), is expanded while being kept in origin symmetry as shown by the broken line in FIG. 8-(a) when the object 240 moves as a whole uniformly toward the probe coils 225 and 226, and is distorted not in origin symmetry as shown by the broken line in FIG. 8-(b) when the object 240 moves other directions. In a case where the flaw detection is carried out by the self-comparison method which disposes two probe coils 225 and 226 close to each other and picks up the impedance difference in comparison with the existing of flaws on the object 240 at the portion thereof corresponding to the probe coils 225 and 226 in close contact with each other, when the object 240 moves uniformly toward the probe coils 225 and 226 as shown in FIGS. 6-(a) and 7-(a), the lift-off of each probe coil 225 and 226 becomes equal to keep the symmetry of detection pattern of flaw signal, thus merely expanding (or diminishing) the signal. On the other hand, when the object 240 moves other directions, the lift-off of one probe coil 225 is reduced to be a pattern in the first quadrant in FIG. 8-(b) and that of another probe coil 226 is enlarged to be a pattern in the third quadrant in the same figure. Thus, the two kinds of signals cannot be similarly corrected merely by measuring the distance between the probe coils and the object, thereby requiring to pick up configuration of each lift-off variation and correct the signal.
Generally, in order to be less affected by the lift-off variation, the probe type eddy current inspection method employs the self-comparison method, so that when a lift-off value for one probe coil just above the flaw is different from that for the other in the same condition, a distortion is produced as shown in FIG. 8-(b). Also, the lift-off variation signal detected by the multifrequency method is not clarified of the absolute value of lift-off because the signal is a difference in lift-off between both the coils.